Electronic flash apparatus

ABSTRACT

Electronic flash apparatus for photographic purposes, wherein a flash tube is discharged to produce the desired light, and the discharge is terminated when sufficient light has been produced. The determination of when to terminate the discharge is accomplished by integrating the light reflected from the subject being photographed. In order to avoid the over-exposure which frequently occurs when apparatus of this general type is used for taking photographs at short distances, there is an impedance incorporated in the discharge circuit in front of the flash tube, and a commutating capacitor is connected to an intermediate tap in this impedance, and to an electronic switch, the commutating capacitor and switch being in parallel with the flash tube and a thyristor which is in series with the flash tube. A resistor bypasses the commutating capacitor. A connection from the junction between the commutating capacitor and the electronic switch goes through a break-down diode and another capacitor to the junction between the flash tube and its thyristor.

United States Patent [1 Schneider [111 3,864,600 1451 Feb. 4, 1975 ELECTRONIC FLASH APPARATUS [75] Inventor: Arthur Schneider, Volkenrode,

Germany [73] Assignee: Rollei-Werke Franke & Heidecke,

Branschweig, Germany [22] Filed: Jan. 22, 1974 [21] Appl. No.: 435,594

Related US. Application Data [63] Continuation-in-part of Ser. No. 307,605, Nov. 17,

[30] Foreign Application Priority Data Feb. 2, 1973 Germany 2305022 [52] U.S.Cl. 315/241 P,315/15l,3l5/159 [51] Int. Cl. H05b 41/32 [58] Field of Search 315/241 P,155,151',159

[56] References Cited UNITED STATES PATENTS 3,519,879 7/1968 Ogawa 315/151 3,818,266 6/1974 Vital et al 315/241 P X Primary Examiner-Nathan Kaufman Attorney, Agent, or FirmStonebraker & Shepard [57] ABSTRACT Electronic flash apparatus for photographic purposes, wherein a flash tube is discharged to produce the de sired light, and the discharge is terminated when sufficient light has been produced. The determination of when to terminate the discharge is accomplished by integrating the light reflected from the subject being photographed. In order to avoid the over-exposure which frequently occurs when apparatus of this general type is used for taking photographs at short distances, there is an impedance incorporated in the discharge circuit in front of the flash tube, and a commutating capacitor is connected to an intermediate tap in this impedance, and to an electronic switch, the commutating capacitor and switch being in parallel with the flash tube and a thyristor which is in series with the flash tube. A resistor bypasses the commutating ,capacitor. A connection from the junction between the commutating capacitor and the electronic switch goes through a break-down diode and another capacitor to the junction between the flash tube and its thyriszor.

4 Claims, 2 Drawing Figures ELECTRONIC FLASH APPARATUS CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my application Ser. No. 307,605, filed Nov. l7, 1972.

BACKGROUND OF THE INVENTION The invention deals with an electronic flash unit for photographic exposure purposes, the unit being provided with a photoelectric measuring device emitting an impulse when a settable amount of the light emitted by the flash tube and reflected by the subject being photographed has been achieved, in order to interrupt the flash tube discharge. A thyristor located in the discharge circuit of the flash tube and ignited (made conductive) at the same time as the flash tube is connected to a commutating capacitor by means of an electronic switch. In some embodiments offlash apparatus of this general type, the thyristor is provided with an ignition device located between the flash tube and the ground connection of the thyristor, there being a resistor in parallel with the thyristor on one hand, and on the other hand a capacitor and a potentiometer circuit at which the control grip voltage of the thyristor is tapped off. Flash units having such circuits are disclosed in my above mentioned application Ser. No. 307,605.

With such flash units, it was found that at the moment of commutation, due to the charging current of the commutating capacitor flowing through the flash tube, an additional light amount is emitted by the flash tube. This additional amount is shown by the shaded area under a in FIG. 2. Especially in the close-up range, when this additional amount of light is large in comparison to the proper amount of light needed for an extremely short exposure time, this leads to a faulty exposure of the photographic film. From the moment at which the photoelectric measuring device measures the required value for a correct exposure, the emission of light should be instantly cut off. But in the prior devices mentioned, instead ofinstantly cutting off the light, the flow of current from the commutating capacitor actually increases the emission'of light from the flash tube, for a brief interval, and so over-exposure results when taking close-up photographs. This additional emission of light caused by flow from the commutating capacitor is not so serious when making'exposures at longer distances, because at greater distances the total amount of light needed for proper exposure is greater, and the additional emission of light caused by flow from the commutating capacitor is small in comparison to the total amount of light needed for the exposure, so the additional or unwanted light is not so detrimental. But at close ranges. it is very detrimental and may cause serious over-exposure.

The object of the present invention is to improve an electronic flash unit of the general type above mentioned, in such a way that the additional or unwanted light emission is eliminated to a large extent.

SUMMARY OF THE INVENTION 7 pacitor and a potentiometer circuit of the ignition de- According to this invention, the above mentioned obvice is connected through a break-down diode and a further capacitor to the junction between the commutating capacitor and the electronic switch.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuit diagram of flash apparatus in accordance with a preferred embodiment of the invention; and 7 FIG. 2 is a diagram or graph of light flux with respect to time, when using flash apparatus for photography at a relatively close-up range or distance.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the preferred form of the present invention. schematically illustrated in FIG. 1, there is a main storage capacitor 1 which, as common in known apparatus of this general type, is charged up to the required operat ing voltage from any suitable source of direct current schematically shown at 2, this being, for example, a d.c. converter. Arranged in parallel with the storage capacitor 1 is a conventional flash tube 3, in series with a thyristor 4 controlling the flash discharge. The cathode of the thyristor is directly connected to the flash tube. An impedance 5 is inserted in the discharge circuit, between the storage capacitor 1 and the anode of the thyristor 4, the impedance preferably being located between the capacitor 1 and the anode of the flash tube 3. A diode 6 is connected around (in parallel with) the impedance 5, to prevent an excessive voltage.

At an intermediate tap of the impedance 5 there is a connection to a commutating capacitor 7 and an electronic switch 9 arranged in series with each other and in parallel with the storage capacitor 1, and likewise in parallel with the flash tube 3 and its thyristor 4. A resistor 10 by-passes the commutating capacitor 7 and the part of the impedance 5 to the right of the tap.

A resistor 11 is arranged in parallel with the thyristor 4, as illustrated. Also a series circuit consisting of the capacitor 12 and the potentiometer circuit composed of the resistors 13 and 14 is in parallel with the thyristor 4 and the resistor 11. The control voltage of the thyristor 4 is'tapped off from the center tap between the resistors 13 and 14 of the potentiometer circuit. An-

other capacitor 15 and a break-down diode 17 are in series with each other, connected between the junction of the elements 12 and 13 and the junction between the elements 7 and 9.

The flash tube 3 is ignited at the desired time (in synchronism with the operation of the shutter of the camera with which this flash apparatus is used) by the conventional ignition device indicated schematically at 18. Such ignition devices are well known in the art, and may be similar, for example, to the ignition device formed by the parts 7, 8, 9, 10 of my above mentioned Pat. application Ser. No. 307,605, or the ignition device formed by the parts 8, 9,10,11, 12 of Krusche and Rohmann Pat. No. 3,585,442, granted June 15, 1971.

The operation of the device is as follows:

The storage capacitor 1 is recharged to the proper operating voltage through the direct current converter 2 or other suitable source of'direct current. The capacitor 15 becomes charged to the same operating voltage, whereas the capacitor 7 remains uncharged on account of the by-pass through the resistor 10. The main electrodes of the flash tube 3 are provided with full operating voltage, whereas the anode and cathode of the thyristor 4 have the same potential.

When the ignition device 18 is operated (as for example by closing the synchronizer switch associated with the shutter of the camera with which this flash device is used) a trigger impulse is produced, which ionizes the discharge path of the flash tube 3, and the ignition of the flash tube is thereby effected. When the flash tube ignites, the potential at the anode of the thyristor 4 suddenly rises. As a result of this rapid increase in potential, a positive impulse is transmitted via the capacitor 12 and potentiometer circuit l3, 14 to the control grid of the thyristor 4. This ignites the thyristor 4, making it conductive through its anode-cathode circuit, so that the entire flash power can now flow through the flash tube. At this stage almost the entire operating voltage from the storage capacitor 1 lies at the flash tube 3. The flash tube emits a light flux corresponding to the curve c of the graph in FIG. 2.

The light emitted from the flash tube 3 is reflected from the subject being photographed, and reaches the light sensitive circuit element of the measuring and integrating device indicated schematically at 8. The measuring device 8 may be of conventional form well known in the art. When this device 8 senses the integrated value of the light required for the necessary correct exposure, it transmits an ignition impulse to the electronic switch 9, which immediately becomes conductive or arcs through.

When the electronic switch first triggers, or becomes conductive, the voltage on the capacitor 7 is still at zero, and as a result the entire voltage occurring at the storage capacitor lies in the left hand half of the impedance 5. This voltage is induced by a converter into the right hand half of the impedance 5, and here produces a commutating current impulse via the capacitor 7, the electronic switch 9, the thyristor 4, and the flash tube 3. The current flowing via the electrode of the thyristor 4 in the direction anode to cathode is thereby at zero, and the thyristor therefore blocks or becomes nonconductive, which stops the flash discharge through the flash tube 3, which is thereby extinguished.

To insure a safe commutation of the thyristor, the anodecathode voltage at the thyristor 4 must remain at zero or negative until all charge carries have been removed from the PN junctions of the thyristor. This socalled vacant time is reduced by applying a negative voltage at the control grid of the thyristor during commutation. The capacitor 15 supplies this negative voltage when the electronic switch 9 is triggered or made conductive, as the capacitor 15 lies directly at the zero potential when the electronic switch is triggered, and its discharge current generates a negative grid voltage via the resistor 14.

As the vacant time of the thyristor is considerably shorter than that of the flash tube 3, the voltage at the anode-cathode path of the thyristor 4 rises during the period until the flash tube 3 is de-ionized. The breaki 4 down diode 17 thereby prevents the rise in voltage generating a positive control impulse via the capacitor 12 'at the control grid of the thyristor 4 as when the electronic switch 9 is closed the control grid is negatively biased around the break-down voltage of the breakdown diode.

A series connection of forwardly directed poled diodes in the direction of the capacitor 12 to capacitor 15 can be used instead of the break-down diode 17, whereby the sum total of their threshold voltages corresponds to the break-down voltage of the break-down diode.

As a result of the improved circuit here disclosed, the light flux emitted from the flash tube 3 immediately drops at the moment of commutation indicated schematically at 1* shown in the graph, FIG. 2, the flux dropping down along the curve b. Thus the additional unwanted light emission represented by the curve a in FIG. 2, which occurs in prior flash devices of this same general type, does not occur when the flash device uses the improved circuit here shown.

What is claimed is:

1. Electronic flash apparatus comprising a storage capacitor, a flash tube and a thyristor in series with each other and in parallel with said storage capacitor. ignition means for igniting said flash tube so that current from said storage capacitor may flow through said tube to create a flash of photographic light therein, and extinction means for stopping flow of current through said tube to stop emission of light therefrom when sufficient light for the photographic purpose has been produced, said extinction means including an impedance located in the discharge circuit between said storage capacitor and said tube, said impedance having an intermediate tap, a commutating capacitor and an electronic switch connected in series with each other and in parallel with said tube and thyristor through said intermediate tap, a by-pass resistor in parallel with said commutating capacitor and a portion of said impedance, a third capacitor and a potentiometer circuit in series with each other and in parallel with said thyristor, the gate of said thyristor being connected to an intermediate point of said potentiometer circuit, and a circuit connection from a junction between said third capacitor and said potentiometer circuit to a junction between said commutation capacitor and said electronic switch, said circuit connection including diode means and a fourth capacitor in series with each other.

2. Apparatus as defined in claim 1, wherein said circuit connection also includes a resistor in parallel with said diode means.

3. Apparatus as defined in claim 1, wherein said diode means is a break-down diode.

4. Apparatus as defined in claim 1, wherein said diode means is a series of forwardly directed diodes faced in a direction from said third capacitor to said fourth capacitor. 

1. Electronic flash apparatus comprising a storage capacitor, a flash tube and a thyristor in series with each other and in parallel with said storage capacitor, ignition means for igniting said flash tube so that current from said storage capacitor may flow through said tube to create a flash of photographic light therein, and extinction means for stopping flow of current through said tube to stop emission of light therefrom when sufficient light for the photographic purpose has been produced, said extinction means including an impedance located in the discharge circuit between said storage capacitor and said tube, said impedance having an intermediate tap, a commutating capacitor and an electronic switch connected in series with each other and in parallel with said tube and thyristor through said intermediate tap, a by-pass resistor in parallel with said commutating capacitor and a portion of said impedance, a third capacitor and a potentiometer circuit in series with each other and in parallel with said thyristor, the gate of said thyristor being connected to an intermediate point of said potentiometer circuit, and a circuit connection from a junction between said third capacitor and said potentiometer circuit to a junction between said commutation capacitor and said electronic switch, said circuit connection including diode means and a fourth capacitor in series with each other.
 2. Apparatus as defined in claim 1, wherein said circuit connection also includes a resistor in parallel with said diode means.
 3. Apparatus as defined in claim 1, wherein said diode means is a break-down diode.
 4. Apparatus as defined in claim 1, wherein said diode means is a series of forwardly directed diodes faced in a direction from said third capacitor to said fourth capacitor. 